The leaked Sinclair Knight Merz report on Auckland's city centre transport options reiterates the dire predictions made in two previous reports - that relying on existing roads as the transport solution for Auckland's burgeoning population is a one way street to gridlock.

Eighteen months ago, council transport planners predicted that without the rail tunnel, Auckland by 2041 would need a four-lane busway running out of the city.

"This would take the entire width of the roadway and effectively stop all general traffic from using those roads."

The independent engineers from SKM have repeated the warning, highlighting Symonds St as the likely four lane exclusive bus highway. They also suggest a trench - is it just me, but the historic Symonds St Cemetery comes to mind - as a possible option.

The report highlights the fact that many downtown streets have already reached capacity as far as buses are concerned. So what hope for the CBD in 30 years when Auckland is home to another million residents and still tunnel-less?

Fanshawe, Customs, Wellesley and Albert Sts are already carrying up to 100 buses an hour during rush hours - Symonds St has 125 an hour - and by 2021, Fanshawe and Wellesley streets will have 150 and Symonds St, 200 hour. In London, the council had to restrict buses numbers in Oxford St in 2010 when the frequency reached 160 as hour. At that stage, average speeds had dropped to 4.3km/h and it was taking 22 minutes to travel the 2.4km distance.

This is the nightmare scenario looming if the nervous Nellies in the government continue to think small and hope more buses is all Auckland needs. What the SKM report also highlights is the shortcomings of relying on the economic modelling alchemy of benefit cost ratios. According to the government formula followed by SKM, the proposed rail tunnel will return only 78c in economic benefit for every dollar spent building it. Even extending the life of the benefits to 60 years - twice the time accepted by government rules - takes predicted benefit to just $1.05.

In the Government's eyes this is an inadequate return - unless, that is, it's a proposed new highway running through National-held electorates.

Any sane bystander would say something must be wrong with a formula that suggests it is better for Aucklanders and the nation to stand aside and watch the Auckland central business district slowly die from congestive heart failure, rather than plan for a bypass operation.

The Auckland Council's chief economist, Geoff Cooper, raised this issue in these pages this month.

One suspects he must have seen an earlier copy of the SKM report before he penned his piece.

Mr Cooper said: "New Zealand policy makers have settled on some of the most short-sighted appraisal methods of the developed world".

He said that if we applied the British framework to the city rail link, the project benefits would be around six times higher and "the resulting benefit-to-cost ratio would likely stoke a great deal of public confidence".

"The hurdle for long-lasting infrastructure in New Zealand is far too high, which erodes public confidence [and] means that short-lived, piecemeal projects are usually given priority," he said.

He highlights that the reason the second harbour crossing and the rail tunnel are assessed as having such poor economic returns is that the government "guidelines for appraisal miss many of the economic benefits".

He says really big projects such as the existing harbour bridge "can fundamentally change the drivers of economic growth by creating changes in land use, enticing population inflows and generally increasing the amount of economic activity ... yet none of these benefits are considered in the economic appraisal of modern transport projects".

As for the exclusion of any economic benefit beyond 30 years, he points to Auckland's 50-year-old bridge and century-old water and rail networks and says it was lucky our forbears didn't use modern day cost benefit guidelines to decide on these projects.

Brian Rudman's first news story was for Auckland University student paper Outspoke, exposing an SIS spy on campus during the heady days of the Vietnam War. It resulted in a Commission of Inquiry and an award for student journalist of the year. A stint editing the Labour Party's start-up Auckland newspaper NZ Statesman followed. Rudman decided journalism was the career for him, but the NZ Herald and Auckland Star thought otherwise when he came job-hunting. After a year on the "hippy trail" overland to London, he spent four years on Fleet St with various British provincial papers. He then joined the Auckland Star, winning the Dulux Journalist of the Year award for coverage of the 1976 Dawn Raids against Polynesian overstayers. He has also worked on the NZ Listener, Auckland Sun, and since 1996, for the NZ Herald as feature writer and columnist. He has a BA in History and Politics.